I don't believe it is fully YouTube's doing. I see lots of new videos without ad interruptions and I don't have a blocker. I really think it is the YouTuber themselves to blame. I think YouTube gives the option to have or not have ads on your video....if you have ads, you get paid a bit for each ad during your video. I've even seen videos where the ads were placed during clear pausing points in videos.....for example, a disc golf tutorial talks about what is going to happen, an ad happens, then the video shows what they described.
So, because I've seen videos without ads and ones where the ads are placed properly for the flow of the video...I really don't think YouTube is to blame. It just shows me that more video posters are wanting to monetize their videos. Maybe YouTube does add advertisements...but it would be strange they do it for some and not all.